1. Field of the Invention
The present invention relates generally to the field of optical sensors used with apparatus for measuring the size and velocity of particles suspended in a fluid medium, and more specifically to an optical waveguide probe having variable gap focusing for transmitting light to and from the measurement zone.
2. Discussion of the Related Art
U.S. Pat. No. 5,094,532, issued Mar. 10, 1992, and entitled xe2x80x9cMethod and Apparatus for Measuring Small Particle Size Distributionxe2x80x9d, to Michael N. Trainer et al., and currently assigned to the assignee of the present invention, teaches a method and apparatus for measuring the size distribution of very small particles suspended in a fluid medium. The apparatus described includes a dynamic light scattering instrument having an optical coupler.
U.S. Pat. No. 5,094,526, issued Mar. 10, 1992, entitled xe2x80x9cIntegrated Optical Waveguide Doppler Velocimeterxe2x80x9d, to Freud et al., and currently also assigned to the assignee of the present invention, describes an optical apparatus that is used to practice the small particle size distribution measurement methods taught by U.S. Pat. No. 5,094,532. U.S. Pat. No. 5,094,526 describes an optical apparatus having a 2xc3x971 optical waveguide splitter that is integral to a probe assembly that forms a part of the Doppler velocimeter. The integrated optical surface waveguide includes a first optical waveguide path that receives an incident beam of light from a light source on one end. The first waveguide path guides the incident beam to a second end that further includes an optical focusing element and a glass rod element permanently secured to the second end. The glass rod is immersed into the suspension medium containing the particles to be measured and conveys the beam to the suspension to irradiate the particle ensemble therein. A second optical waveguide path is optically coupled to the first waveguide path for receiving the scattered light from the particles as well as the non-scattered light Fresnel reflected from the face of the glass rod and guides both to the other end thereof. A detector receives the scattered and non-scattered light from the second waveguide and converts it to an electrical indicative of a Doppler frequency shift. The integrated optical waveguide has advantages over fiber optic couplers due to its more rugged nature, its reduced susceptibility to environmentally induced optical phase noise, its polarization stability and its favorable signal-to-noise ratio characteristics.
In the aforementioned construction the focusing element is secured to the first waveguide path by means of an index matching epoxy. The glass rod element is similarly fixed to the focusing element by means of a similar index matching epoxy. The epoxy is chosen to match as closely as possible the index of refraction of the first and second waveguides in order to minimize any reflection between interfacing surfaces.
This construction requires a great degree of precision in the assembly of the focusing element and glass rod to the waveguide. In order to gain the maximum benefit from the focusing element""s advantages in increasing the field of view, the focusing element and glass rod must be sized to allow the focus of the beam to fall substantially on the face of the rod. Therefore, the design and assembly of these elements requires consideration to the size of the elements, gradient index of the focusing element, and the gap of the epoxy layers used in attaching the elements to each other and to the waveguide. Due to the fixed spatial relationships between the elements and the waveguide, a considerable degree of engineering effort, skill and time is required in the design and assembly of such an optical probe in order for it to provide optimal performance within its intended use.
Therefore, it is an object of the present invention to provide an optical waveguide probe with an optical focusing assembly that can be easily attached and manipulated in order to provide the optical probe with optimal performance characteristics without the need for consideration of the physical and operational factors of the focusing elements.
It is another object of the invention to provide a variable gap focusing structure for an optical waveguide probe that greatly reduces the complexity and precision of its manufacture and assembly.
In accomplishing these and other objects, there has been provided, in accordance with the present invention, an adjustable focusing assembly for an optical waveguide probe. The optical probe includes a housing and an optical waveguide having a termination end for the emission and reception of light images mounted within the housing, with the termination end proximate a housing first end. The focusing assembly includes a focusing element having a first and a second end mounted within a holder member. The holder member is secured to the housing first end with the focusing element first end adjacent to, and optically coupled to, the optical waveguide termination end. A window having an inner and an outer surface is mounted on a body member with the window inner surface in a facing and spaced relationship to the focusing element""s second end. The body member further includes a cowled portion extending from the periphery of the body member on a side opposite the window, arranged to engage surfaces on the periphery of the holder member, thereby defining a cavity between the focusing element second end and the window inner surface. The body member is adjusted increasing or alternatively decreasing the cavity until the light images from the focusing element second end are focused on the window outer surface. Upon establishment of the focus, the cowled portion is cemented to the holder member, fixing the focusing assembly to the optical probe. Alternatively, the cavity is filled with an epoxy cement having an index of refraction that closely matches the index of refraction of the optical waveguide. After adjustment of the body member to focus the light images on the window outer surface, the focusing assembly is fixed to the optical probe by curing the index matching epoxy.
A second embodiment for accomplishing the objects is also provided that includes a focusing assembly having a focusing element with a first and second end mounted within a body member. A window having an inner and an outer surface is mounted on the body member with the window inner surface resting against and in contact with the focusing element first end. The body member further includes a cowled portion extending from the periphery of the body member on a side opposite the window, arranged to engage the housing first end, thereby defining a cavity between the termination end and the focusing element second end. With the cowled portion mounted on the housing, the focusing element second end is in a spatial and axial alignment with the optical waveguide termination end. The body member is adjusted until the light images on the termination end are transferred to the focusing element and focused on the window outer surface. Upon establishment of the focus, the cowled portion is cemented to the housing, fixing the focusing assembly to the optical probe.
Alternatively, the cavity is filled with an epoxy cement having an index of refraction that closely matches the index of refraction of the optical waveguide. After adjustment of the body member to focus the light images on the window outer surface, the focusing assembly is fixed to the optical probe by curing the index matching epoxy.